1. FIELD OF THE INVENTION
This invention relates to pneumatic isostatic forging of sintered compacts, including pneumatic isostatic forging of a rolling cutter adapted for use in a steel tooth rolling cutter drill bit utilized for drilling bore holes in the earth, the rolling cutter having a layer of wear resistant material, such as a hardmetal facing.
2. DESCRIPTION OF THE RELATED ART
It is well known to make sintered products by compacting metal powder, such as a plurality of iron, or steel alloy, particles in a die to form an unsintered compact, typically called a xe2x80x9cgreenxe2x80x9d compact, and then heating the green compact to a suitable temperature for a time sufficient to effect solid state bonding, or sintering, of the particles to each other. One type of compaction process is an isostatic process using a gas pressing medium known as the PIF process, which stands for pneumatic isostatic forging.
The PIF process has been used to densify sintered compacts wherein the sintered compact is encased in a shell which seals its outer surface against penetration of the gaseous pressing medium into the interior of the sintered compact. The sintered compact is then heated back up to the sintering temperature and then may be surrounded by, and subjected to, pressing gas pressures sufficiently high (up to 60,000 PSI) as to densify the sintered compact. The sealing shell make take several forms, as are known in the prior art, including placing the compact in an evacuated thin flexible sheet metal can or mold; or applying a sealant, such as molten glass, electroless nickel, or an oxide sealant grown in situ on the surface of the compact to seal the surface pores.
It is also known to form densified sintered products which have a layer of wear resistant material to a portion of the external surface of the sintered product. An example of such a product is a rolling cutter adapted for use in a steel tooth rolling cutter earth boring bit utilized for drilling bore holes in the earth for the minerals mining industry. In the production of such rolling cutters, hardmetal inlays or overlays are employed as wearing and deformation resistant cutting edges and faying surfaces. These typically comprise composite structures of hard particles in a more ductile metal matrix. The hard particles may be metal carbides, such as either the cast WC/W2 eutectic or monocrystalline WC, or may themselves comprise a finer cemented carbide composite material. The hard particles which could be used include tungsten carbide, tungsten carbide/cobalt, titanium carbide, and commercially available ceramic carbides.
A major problem in forming densified sintered products, such as a rolling cutter having a layer of wear resistant material on it, utilizing a PIF, or pneumatic isostatic forging, process, is that the PIF process typically requires a relatively long period of time to sinter the green compact. This long sintering period, typically at an elevated temperature of approximately 2000xc2x0 F. will, it is believed, either destroy, or at least severely damage, any wear resistant material placed on the unsintered green compact, such as the hard particles previously discussed. In the case of a green compact formed of steel alloy powder and a tungsten carbide wear resistant layer, the prolonged heating, at elevated temperatures, leads to the steel alloy attempting to draw out, or suck out, the carbon atoms from the tungsten carbide wear resistant layer, thus either destroying, or severely damaging, the wear resistant layer. Additionally, some of the prior art techniques for sealing the outer surface of the sintered compact in the PIF process may have some disadvantage for some types of products. For example, additional manufacturing steps may be necessary to remove the sealing material, such as molten glass, nickel or oxide sealant, for the end product to be used. Additionally, such sealing materials could be a contaminant to some sintered compacts.
Accordingly, prior to the development of the present invention, it is believed that there has been no method of forming a densified sintered product from a metal powder, particularly when the sintered product includes a layer of wear resistant material thereon, by a pneumatic isostatic forging process, which: does not destroy, or severely damage, the wear resistant material; does not require additional manufacturing steps to remove a sealing material; and does not contaminate the sintered product with a sealing material. Therefore, the art has sought a method of forming a densified sintered product by a pneumatic isostatic forging process, which: does not destroy, or severely damage, a layer of wear resistant material that may form a part of the sintered product; does not require additional manufacturing steps to remove a sealing material; and does not contaminate the sintered product with a sealing material.
In accordance with the invention, the foregoing advantages have been achieved through the present method of forming a densified sintered product from a first metal powder, the powder having a first melting temperature. The present invention includes the steps of: compacting the first metal powder in a die to form an unsintered compact having an external surface; applying a layer of a second metal powder to the external surface of the unsintered compact, the second metal powder having a second melting temperature, the second melting temperature being lower than the first melting temperature; heating the unsintered compact to a temperature greater than the second melting temperature and less than the first melting temperature, to form a sintered compact of the first metal powder and to melt the second metal powder to form a thin glaze of the melted second metal powder on the external surface of the sintered compact; and pneumatically isostatically forging the sintered compact. Another feature of the present invention may include the step of providing a layer of wear resistant material to a portion of the external surface of the unsintered compact prior to applying a layer of the second metal powder.
A further feature of the present invention is that the densified sintered product to be formed may be a rolling cutter adapted for use in a steel tooth rolling cutter earth boring bit. Another feature of the present invention is that the densified sintered product to be formed may be a component of a drag type earth boring bit. The wear resistant material may be a hardmetal composite structure which includes a plurality of hard particles. In accordance with the invention, the hard particles may include tungsten carbide, titanium carbide, and/or ceramic carbide. The first metal powder may be a powder of steel alloy particles, and the second metal powder may be an iron powder. A further feature of the present invention is that the heating of the unsintered compact may be done at a temperature of approximately 2050xc2x0 F.
In accordance with another aspect of the present invention, the foregoing advantages may also be achieved in the present method of forming a densified sintered product from a metal powder, having a first melting temperature. In accordance with this aspect of the present invention, the method includes the steps of: compacting the metal powder in a die to form an unsintered compact having an external surface; applying a layer of a metallic material to the exterior surface of the unsintered compact, the metallic material having a second melting temperature, the second melting temperature being lower than the first melting temperature; heating the unsintered compact to a temperature greater than the second melting temperature, and less than the first melting temperature, to form a sintered compact of the metal powder and to melt the layer of metallic material to form a thin glaze of the melted material on the external surface of the sintered compact; and pneumatically isostatically forging the sintered compact. A further feature of this aspect of the present invention is that the layer of material is formed of a metallic material that upon melting becomes integral with the sintered compact.
In accordance with another aspect of the present invention, the foregoing advantages may also be achieved in the present volume reduction mandrel, for use with at least one sintered compact to be placed in a pressure vessel used in a pneumatic isostatic forging process, wherein the pressure vessel has a hollow body member and an inner cavity, and the at least one sintered compact has an exterior shape. In accordance with this aspect of the present invention, the volume reduction mandrel includes: a mandrel body member having an exterior surface and an interior cavity, the interior cavity closely conforming to the exterior shape of the sintered compact, the interior cavity being adapted to slidingly receive therein the sintered compact; and the mandrel body member, when disposed within the inner cavity of the pressure vessel, provides a small clearance between the exterior surface of the mandrel body and the inner cavity of the pressure vessel and the mandrel body member substantially fills the inner cavity of the pressure vessel. Another feature of this aspect of the present invention is that the volume reduction member may include a closure member for the mandrel body member. An additional feature of this aspect of the present invention may include a plurality of mandrel body members, each mandrel body member being adapted to contain a sintered compact, the plurality of mandrel body members substantially filling the inner cavity of the pressure vessel.
The method of forming a densified sintered product from a metal powder, in accordance with the present invention, when compared to previously proposed prior art methods of forming densified sintered products, has the advantages of: not destroying, or severely damaging, the layer of wear resistant material which may be included in the sintered product; not requiring additional manufacturing steps; and not contaminating the sintered compact.
The volume reduction mandrel, in accordance with the present invention, when compared to previously proposed prior art apparatus for use in a pneumatic isostatic forging process, has the advantage of permitting the high pressure gas, which is being injected into the isostatic forging pressure vessel, to be disposed within as small a volume of space as possible.